Line circuits function to interconnect a telephone station set with a business communication system. The telephone station set is connected to the line circuit via a communication pair which consists of a TIP conductor and a RING conductor. The line circuit provides a number of functions which are collectively termed the BORSHT function: battery, over voltage, ringing, supervision, hybrid, transmission.
The ringing function is ordinarily provided by an electromechanical relay, the contacts of which are connected in series between the RING conductor of the communication pair and the remainder of the line circuit. The electromechanical relay uses a normally closed contact to connect the RING conductor of the communication pair to the remainder of the line circuit and a normally open contact to connect the RING conductor of the communication pair to a source of ringing voltage. Thus, the electromechanical relay, when operated to provide the ringing signal, interrupts the connection between the RING conductor of the communication pair and the line circuit via opening the normally closed contact and instead connects the RING conductor to a source of ringing voltage via closing the normally open contact.
Electromechanical relay contacts have the advantage of presenting a very low series impedance when closed and a very high open circuit impedance when open. The contacts of the ring relay present a negligible impedance between the RING conductor and the remainder of the line circuit while the ring relay is not operated and the RING conductor is disconnected from the source of ringing voltage by an impedance that is virtually an open circuit. The disadvantage of electromechanical relays is that they occupy a significant amount of space on the line circuit board.
An alternative to the electromechanical relay is the use of a semiconductor switch to provide the ring relay function. The difficulty with semiconductor switches is that they present a small but not insignificant series impedance for a closed contact. This series impedance disrupts the longitudinal balance of the line circuit. In addition, the normally open contact of the semiconductor switch incorporates a parasitic capacitance which couples the ringing voltage to the RING conductor when the line circuit is not in the ringing state. This coupled ringing signal presents an element of noise on the communication pair which is unacceptable.
Thus, electromechanical relays provide an ideal switching function but require a significant amount of space while the semiconductor switches do not require much space but provide an imperfect switching function which adversely affects the performance characteristics of the line circuit.